The present invention relates to the field of towed microwave decoys and more specifically, to an improved system where a decoy is towed behind a high speed aircraft and is provided with a wideband signal for radiation both forward and aft of the aircraft. Among the valuable features of the invention are the cables employed for towing the decoy from a high speed aircraft, the efficient and reliable manner in which the wideband signal is provided to the decoy, the compact and lightweight structure created for the whole system and the extremely rapid system deployment which may be obtained. The deployment of a decoy, such as an antenna, from an aircraft while travelling at high speed involves releasing the antenna behind the aircraft and providing for the antenna to reach a desired, and relatively stable, location relative to the aircraft. The present invention relates to a rapid deployment system and the various system components specifically designed for rapid deployment.
Towed systems have been employed in various applications which call for the towing of an element of the system, generally a decoy in the form of an active transmitter from an aircraft. Systems such as this pay out a cable from a reel, a procedure which may consume from ten to sixty seconds for a cable length of about 100 meters. The use of a reel was adopted because its controlled payout speed was believed to be necessary to avoid cable damage which would be encountered if the decoy were simply dropped from the aircraft and allowed to xe2x80x9cfallxe2x80x9d to the end of the cable. Prior attempts to avoid cable damage have focussed either on controlled payout speed or increased strength for the cable. These approaches failed to provide acceptable results. The controlled payout approach has not resulted in rapid deployment. The increases in cable strength have had another problem. It has been found that increases in cable strength result in increased cable weight. However, the increased weight results in a greater mass reaching the xe2x80x9ctowingxe2x80x9d end of the cable and thus requires more strength and an even larger cable diameter. At supersonic speeds, the extra diameter needed to add strength results in such a substantial increase in weight that it is not feasible to solve the strength problem in this manner.
It is desirable to substantially shorten the payout time, for instance to less than 3 seconds. This appears to require abandonment of the reel-type dispenser which is not capable of such rapid payout. Another disadvantage of reel-type dispensers is the weight of the dispensing apparatus. In aircraft of the type having a need for a towed decoy, it is commonly desired to reduce weight to a minimum. Thus, elimination of the reel-type dispenser is desirable.
One possible solution involves employing an elastic strength member in the cable which allows a temporary elongation of the cable as it reaches its full payout. This approach has been found useful for certain applications because the temporary stretching of the cable allows the towed element to accelerate over a somewhat greater period of time than is possible with non-reinforced cable. This approach has however been found unworkable for supersonic applications. The cable, if initially made of a small diameter, stretches too much and actually breaks when supersonic speeds are encountered. Adding diameter to the cable results in greater strength and weight and reduces the amount of elongation at the end of cable payout. The reduction in the amount of elongation is accompanied by a shortened acceleration period (it takes less time for the cable to reach a fully stretched length when the cable is thicker and stretches less) and greater acceleration forces on the cable. As a result, the cable is still subject to breakage in supersonic deployment situations.
According to one aspect of the invention, it is possible to employ drop-type dispensing of the towed element at supersonic speeds and without encountering detrimental cable breakage. It has been found that this can be accomplished through the use of a new design of lightweight cable, even for supersonic deployment. According to this aspect of the invention, a cable design has been developed which becomes permanently elongated as a result of the drop-type deployment, this elongation acting to absorb a substantial portion of the energy of the towed element. The use of a central strength member of stainless steel is one approach which has been found to provide reliable operation.
Another aspect of the invention calls for the provision of a conductive coating around the central strength member and a dielectric coating around at least a portion of the cable""s conductive coating. For subsonic speeds it is possible to obtain reliable drop-type operation through the use of an elastic strength member with a conductive coating thereover. For supersonic speeds an energy absorbing strength member avoids breakage problems.
The cable design is compatible with propagation of a surface wave along the length of the cable. This type of waveguide is known as a Goubau line. Reference is made to xe2x80x9cSurface Waves and Their Application to Transmission Linesxe2x80x9d by Georg Goubau, Journal of Applied Physics, page 1119 et seq., volume 21, November, 1950 for a description of the Goubau line. Additional references which describe the propagation of surface waves along a line include, xe2x80x9cSingle-Conductor Surface-Wave Transmission Linesxe2x80x9d by Georg Goubau at page 619 of the June 1951 Proceedings of the I.R.E.; xe2x80x9cOn the Excitation of Surface Wavesxe2x80x9d by Georg Goubau at page 865 of the July 1952 Proceedings of the I.R.E.; and xe2x80x9cDesign of Cylindrical Surface Waveguides With Dielectric and Magnetic Coatingxe2x80x9d by T. Bercelli at page 386 of the March 1961 Monograph No. 436E of the Institution of Electrical Engineers. U.S. Pat. No. 4,730,172 entitled xe2x80x9cLauncher for Surface Wave Transmitter Linesxe2x80x9d describes an in-line signal launcher.
While these references describe the use of Goubau lines for surface wave propagation, there is no indication that a line can be employed to tow a decoy. Moreover, there is no indication as to how a cable can be made which will withstand the stress associated with xe2x80x9cend-of-payoutxe2x80x9d in a drop-type deployment. The present invention provides a Goubau line which is suitable for use as a xe2x80x9cdrop-typexe2x80x9d payout cable for a towed element. The previous attempts to provide a high quality decoy, towed behind a high speed aircraft, have been limited to slow speed payout, restricted beamforming, restricted power and generally lossy transmission, associated with coaxial transmission lines. According to the present invention, there is no need to employ slow payout, there is no need to limit the beamforming options, the available power is greatly improved and the transmission of the signal to the towed element is very efficient.
The invention makes it possible to employ a drop-type deployment of the towed element using a right angle feed arrangement. In previous approaches to signal transmission onto a Goubau line, it has been satisfactory to rely on certain complicated and difficult to manufacture signal feed arrangements. A signal feed used in a prior approach is that disclosed in xe2x80x9cA UHF Surface-Wave Transmission Linexe2x80x9d, by C. E. Sharp and G. Goubau, PROCEEDINGS OF THE I.R.E., January 1953, pages 107-109. Signal feeds of this type, even though representing themselves as being sufficiently strong for their intended applications, are not adequate to withstand the stresses encountered in the environment of the invention.
According to the right angle feed feature of the invention, it is possible to feed the signal onto the line without concern for the strength of the feed arrangement. The feed arrangement is implemented without exposing the feed structure to the axial load imposed on the cable.
It is an object of the invention to provide the necessary apparatus for the implementation of a drop type towed element system where the towed element may include a radiating antenna.
It is another object of the invention to provide a cable which is capable of withstanding the stresses encountered in a drop type towed element deployment.
It is another object of the invention to provide a signal feed arrangement that can be employed regardless of the axial strength requirements of the cable onto which the signal is to be fed.